What is the purpose of "int mask = ~0;"?

Question

I saw the following line of code here in C.

 int mask = ~0;

I have printed the value of mask in C and C++. It always prints -1.

So I do have some questions:

• Why assigning value ~0 to the mask variable?
• What is the purpose of ~0?
• Can we use -1 instead of ~0?

Answer 1

It's a portable way to set all the binary bits in an integer to 1 bits without having to know how many bits are in the integer on the current architecture.

Answer 2

C and C++ allow 3 different signed integer formats: sign-magnitude, one's complement and two's complement

~0 will produce all-one bits regardless of the sign format the system uses. So it's more portable than -1

You can add the U suffix (i.e. -1U) to generate an all-one bit pattern portably¹. However ~0 indicates the intention clearer: invert all the bits in the value 0 whereas -1 will show that a value of minus one is needed, not its binary representation

¹ because unsigned operations are always reduced modulo the number that is one greater than the largest value that can be represented by the resulting type